Xylan, a major component of plant hemicellulose, is a polymer of D-xylose linked by B-1,4-xylosidic bonds. Xylan can be degraded to xylose and xylo-oligomers by acid or enzymatic hydrolysis. Enzymatic hydrolysis of xylan produces free sugars without the by-products formed with acid (e.g. furans).
There are currently four major applications for xylanses; 1) enzymatic breakdown of agricultural wastes for production of alcohol fuels; 2) enzymatic treatment of animal feeds to release free pentose sugars; 3) manufacturing of dissolving pulps yielding cellulose; and 4) bio-bleaching of wood pulps. [Detroym R. W. In: Organic Chemicals from Biomass, (CRC Press, Boca Raton, Fla., 1981) 19-41.; Paice, M. G., and L. Jurasek. J. Wood Chem. Technol. 4: 187-198.; Pommier, J. C., J. L. Fuentes, G. Goma. Tappi Journal (1989): 187-191.; Senior, D. J., et al., Biotechnol. Letters 10 (1988):907-912.]
The pulp and paper industry is using xylanase compositions in the bio-bleaching process to enhance the brightness of bleached pulps, to decrease the amount of chlorine used in the bleaching stages, and to increase the freeness of pulps in the recycled paper process. [Eriksson, K. E. L., Wood Science and Technology 24 (1990); 79-101.; Paice, M. G., R. Bernier, and L. Jurasek, Biotechnol. and Bioeng. 32 (1988): 235-239.; Pommier, J. C., J. L. Fuentes, and G. Goma, Tappi Journal (1989): 187-191.]
Kraft pulping, a process widely used in the pulp and paper industry, involves the alkaline sulfate cooking of pulp to remove 95% of the lignin. The remaining 5% of lignin gives the pulp a dark brown color which has the tendency to darken in UV light or by oxidation. In order to obtain a white pulp for high quality paper, the brown color is removed by a multi-stage bleaching process using chlorine and/or chlorine dioxide.
Presently, there is much concern about the environmental impact of the chemicals generated from the bleaching process. Enzymes can aid in the removal of lignin from the pulp without any harmful side products. Reports show that lignin in wood is linked to xylan, possibly through an arabinose side chain. [Eriksson, O., et al., Wood Sci.Technol. 14 (1980); 267.; Takashi, N., and T. Koshijiima, Wood Sci.Technol. 22 (1988); 177-189]. By hydrolyzing the xylose-xylose bonds (xylanase, xylosidase) and arabinose-xylose bonds (arabinofuranosidase), a greater release of lignin occurs during bleaching. Thus, by enzymatically treating the pulp prior to bleaching the amount of active chlorine needed would in turn decrease. [Viikari, L., et al., Proceedings of the 3rd International Symposium on Biotechnology in the Pulp and Paper Industry (1986); 67.]